The fast mechanical response of the mammalian auditory outer hair cell is believed to be the cellular basis of the positive feedback mechanism required for the fine tuning process of the hearing organ. We showed that the fast mechanical response of the cell is membrane potential dependent and that a simplified membrane model can describe the elastic property of the cell. We also found that the membrane capacitance of the cell is dependent on membrane tension as well as the membrane potential. This finding indicates that the fast motility is based on conformational changes in a membrane molecule. These observations led to a theoretical model of the cell motility, which predicts the force generation of 0.1 nN/mV for a single cell which agrees with a value 0.2 nN/mV estimated from an in vivo study. We also demonstrated that the lateral wall as well as the stereocilia, can be a mechano-receptor. The marginal cell of stria vascularis is associated with the production of the endocochlear potential and potassium-rich endolymph, which facilitates the mechano-electrical transduction in the sensory hair cells. With the whole-cell recording method, we found that an inward current of the marginal cell is blocked by a micromolar concentration of amiloride. The dose is consistent with amiloride-sensitive sodium channels. Immunocytochemical studies showed that immunoreactivity to antibodies raised against sodium channels from the bovine kidney are distributed in marginal cells as well as vestibular dark cells.